The evolution of these seapeople is really cool to see!
I would suggest more difference betwen male and female characters-especially head design.
Concerning the tail, the first and the fourth from the initial tail sketches were my favorites.
The legs-the first post with the body ideas... The third iteration in the top row is something i haven't seen before and i think it's brilliant! They could use the feet as fins in the water but pull their fingers together to walk/run on land.
If they will fight humans, consider their fight style and design the armor that would be needed for that kind of battles (in tight formation, hunter type, ranged or melee etc.)
You might add something that would sugest they are in aggressive stance-like the collar on beta fish.
I hope this is the kind of feedback you want and is helpful.
Anyways, this thread looks great so far, keep it up!